Nowadays, multi-standard broad band transceiver is an emerging topology in cellular telecommunication systems. Moreover, high performance is required with several standards with stringent specification among the targeted applications. The subject is further complicated by requirements on more compact form factor and on design approaches toward more integration. Moreover, Complementary Metal Oxide Semiconductor, CMOS low quality factor components are limiting the performance of many circuit techniques. Consequently new architectures and circuit techniques must be explored.
To address some of the mentioned issues in the wireless transceivers, In-phase and Quadrature-phase, I/Q modulator and Harmonic Rejection Mixers, HRM have recently gained attention. Indeed the vast majority of transceivers fall into I/Q modulator based receivers and transmitters. These modulators use In-phase and Quadrature-phase mixers. I/Q mixers address the problem of maximizing information transmission in a limited bandwidth by allowing the operator to modulate both the in-phase and quadrature phase components of a carrier simultaneously, doubling the information density.
In order to drive an I/Q mixer, in one approach, a divider able to create 25% duty cycle signals from an externally supplied clock at 2*fLO is often used, where fLO is the frequency of local oscillator, LO signal input to the mixer. A divide-by-2 circuitry which generates 4 phase signals with 25% duty cycle is an example of a 4-phase signal generator.
Conventional switching mixers introduce relatively large harmonics at multiples of Radio Frequency, RF signal input and/or RF signal output frequency of transceivers, demanding filtering in specific locations of the signal chain in the transceivers. Indeed the aforementioned 4-phase signal generator in an I/Q modulator has a harmonic suppression/conversion effect. The fact that the I/Q mixer samples with the quadrature LO signals results in image rejection for each harmonic, however the remaining odd harmonics on the antenna port will reradiate and/or down convert to the baseband. The reason of this is because even in receivers, passive mixers which are bidirectional type of mixer, will back up-convert baseband signals present on the capacitors in the baseband circuits during each LO cycle. In other words, out of band interferers at odd harmonics of the LO signals will be down converted to baseband. Odd harmonics reradiate as well from antenna in receivers after back up-conversion and radiate in transmitters after conversion. Hence suppression of the out of band interferers and suppression of direct radiation of odd harmonics by filtering or HRM will be essential for transceivers performance. However, substantial cost saving could be achieved by removal or reducing the filter's performance. In wideband systems the issue is more important to tackle. This is because e.g. in a wideband low noise amplifier, LNA in the receiver, interferers will be amplified with lower selectivity in the signal chain. It is thus important to suppress the 3rd and higher order harmonics.
The HRM based transceivers address bandwidth, selectivity and filtering in the emerging wireless communication systems. Nowadays, almost all HRM based transceivers use I/Q based type of modulators for purpose of image rejection and bandwidth efficiency.
Among the different orders of the harmonics, the 3rd and 5th order harmonics are critical to supress. Harmonic rejection mixers up to 5th order may be realized as 8-phase mixers driven by 8 phase signals for suppression of 3rd and 5th harmonics.
In addition to the applications mentioned above, where 4-phase signals and 8-phase signals are suitable for the I/Q mixers and 8-phase HRMs in the transceivers, higher order poly-phase signals, e.g. 16-phase, 32-phase etc., or higher order frequency dividers, e.g. divide-by-2, divide-by-4 etc., may be applicable in higher order HRM transceivers or for other applications in other electronic devices, e.g. oscillator scopes, measurement instruments etc.